1. Field of the Invention:
This invention relates to a voltage-tuned, active filter for delivering an output signal having a constant percentage band width of a tuned frequency from and input signal having a wide range of frequencies.
2. Descripiton of the Prior Art:
Heretofore electrical filters have been categorized for some purposes as passive filters and active filters. Passive filters in general are those filters which do not require external power. R/C filters, L/C filters of the passive type are well known. Active filters are those which employ external power and which normally do not alter the signal which is experiencing filtration.
Prior art filters, active and passive, have relied upon fixed electrical circuit components such as resistors, capacitors and inductors. Where the circuit components are fixed, the tuned frequency of the resulting filter is also fixed. In order to provide for variable tuning of the filter center frequency, certain of the filter elements have been made variable. Thus by adjusting the resistance of certain resistors, the capacitance of certain capacitors and the inductance of certain inductors, some variation of filter response frequency has been achieved. The range of such variation has been limited. Accordingly, multiple separate complete filter circuits have been used to provide a filtering response over small segments of a relatively wide spectrum.
There exists a need for a voltage-tunable filter having a constant percentage band width. Such filter should be accurately and precisely tunable over a wide spectrum of input signal frequencies.